The present invention relates to foamed products. More Particularly, the present invention relates to foamed products comprising a plurality of layers of a foamed thermoplastic composition having a low density.
Methods for producing foams comprising a plurality of polymer layers are known. U.S. Pat. No. 4,053,341 discloses a process for producing a laminate comprising a plurality of cross-linked polyethylene foam layers having an anisotropic pore structure in at least one internal layer. The internal layer comprises a cross-linking agent and a large amount of blowing agent, whereas the external layers comprise a cross-linking agent and a small amount of blowing agent. The layers are welded at 160xc2x0 C. and heated at 190xc2x0 C. to 250xc2x0 C. to foam the structure. A disadvantage of this process is that it requires two steps: first welding and cross-linking the extruded plastic layers, then foaming at a higher temperature. Further, the process is limited to the production of a multi-layer foam sheet with limited cross-section.
U.S. Pat. Nos. 4,107,247 and 4,206,165 and Canadian Patent 1,100,726 describe a method for the manufacture of a plastic sandwich element in which a first extruder produces a foamed cellular plastic at a defined temperature. A second extruder produces a solid plastic at a temperature at least 14xc2x0 C. (25xc2x0 F.) hotter than the temperature of the foamed cellular core to promote foaming of the core. The resulting sandwich is extruded through an extrusion die. A drawback of this process is that there is only one single central foam layer confined between two solid plastic films. Further, the resulting foam has comparatively poor insulation and dimensional stability properties.
U.S. Pat. No. 4,773,448 discloses a process for producing a plastic pipe with a hard outer shell lined with an inner shell of soft resilient plastic foam with a smooth axial passageway adapted to be a conduit for liquid. An outer PVC shell is extruded and an inner polyethylene foam material is simultaneously extruded inside the PVC tube. A disadvantage of this process is that the maximum cross-section is the size of the tube. Further, because of the restrained foaming it is expected that the resulting foam exhibits a very high foam density.
JP 50-107067 discloses a process for producing a thermoplastic foam having a non-foamed skin layer by using co-extrusion. A polyethylene foam having a polypropylene skin layer and a polystyrene foam having a polyethylene skin are prepared. A disadvantage of this process is that there is only one single central foam layer confined between two solid skin layers.
Japanese Pat. No. 48-5100 discloses a process for producing a multi-layer polystyrene foam having different foam densities, for example, high density outside, low density inside, and a reinforcement material in between. The material is used as pillars in homes, exhibiting desired bending and strength properties. A disadvantage of this process is that only foams having a comparatively high density of 200 kg/m3 can be manufactured.
Japanese Pat. No. 50-50473 discloses a laminated molded product composed of a foam layer and a non-expanded thermoplastic layer. Both layers are molded into a desired shape. The lamination can take place separately using two extruders and two dies, or simultaneously using two extruders and one multilayer die. A disadvantage of the production process described therein is that only products having a small cross-section and a high density are obtained.
Laid-open Japanese Patent Application H7-112480 discloses a rectangular molded resin product composed of a hollow multilayer resin filled with a resin foam inside the hollow multilayer. The material for the hollow multilayer is polypropylene. The foam is polypropylene or polyurethane. The disadvantage of this manufacturing process is that the molded resin product can have one single foam layer only. Moreover, only products exhibiting a restricted rectangular cross-section and a high foam density can be produced.
U.S. Pat. No. 3,694,292 describes an extrusion head for producing a multilayer blown tubular film.
Generally it has to be noted that considerable problems arise with regard to corrugation of single layers when producing multilayer foams. Particularly, when producing multilayer foams which exhibit three or more foam layers, the foaming of the center layer or layers takes place in a different environment as compared to the outer layers.
Thus, there is a great need to provide a multilayer foam having excellent insulating and dimensional stability properties at a low density. It would also be desirable to provide a method for producing a multilayer foam which is simple, inexpensive and environmentally acceptable and results in products having good physical properties.
In one aspect, the present invention thus pertains to a method for producing a polymer or copolymer multilayer foam, comprising the steps of: (a) providing at least one foamable composition comprising at least one polymer or copolymer and a blowing agent formulation, (b) extruding the composition through a die having a plurality of orifices, (c) foaming the extruded composition at a foaming temperature which is above the glass-transition temperature or the melting temperature of the polymer composition, (d) maintaining the foaming product at an elevated temperature for a sufficient period of time to obtain adhesion between individual foam layers, and (e) allowing the foamed product to cool, characterized in that a foamed product is obtained comprising a plurality of adherent foam layers, wherein at least one of these foam layers extends across the whole breadth of the foamed product.
In a further aspect, the invention relates to a polymer or copolymer multilayer foam, obtainable by the method as described above. This product has excellent properties with regard to thermal insulation, dimensional stability, water diffusion resistance and water vapor permeation resistance. Furthermore, the foam of the present invention has a smooth, uniform surface equivalent to current monolithic foams. This preferred appearance is obtained on both sides of the final foam product without any additional surface-modifying operations.
The foamable composition comprises at least one polymer or copolymer. The composition may comprise a single polymer, a single copolymer, mixtures of polymers, mixtures of copolymers or mixtures of polymers and copolymers.
According to the present invention a multilayer foam is composed of a plurality of polymer or copolymer foam layers. These foam layers may be comprised of the same or of different foamable compositions. The multilayer foam comprises at least two and preferably at least three foam layers. At least one of these foam layers is a homogeneous foam layer, that is, a monolithic foam part extending across the whole breadth of the foam. This homogeneous or monolithic foam layer is obtained by extruding the foamable composition through an orifice which extends across the whole breadth of the die, for example, a slit, or through a continuous orifice, for example, a circle.
The present invention relates to such multilayer foams composed of a variety of such monolithic layers, but also to such foams comprising at least one layer extending across the whole breadth of the foam and at least one layer which is comprised of a plurality of individual coalescent foam strands.
By means of the process for the production of multilayer foams according to the invention, it becomes possible to obtain foams having a greater overall thickness, or to produce them in a simpler manner. Moreover, the resulting multilayer foams have an oriented structure and offer an improved thermal insulation performance at the same density or thickness compared to monolithic foams. Furthermore, they exhibit an excellent dimensional stability and strength with similar or, more preferably, substantially equivalent water permeation resistance and water absorption properties as compared to monolithic foams.
It has been discovered that polymer or copolymer multilayer foams, for example, multilayer foams consisting of a plurality of monolithic layers or multilayer foams having one or more layers consisting of a plurality of individual coalesced strands can be produced on-line without using adhesive additives or any additional mechanical devices to enhance the adhesion between strands or layers.
The multilayer foam can be made from any foamable polymer or copolymer, for example, alkylene aromatic polymers, such as polystyrene or styrene-based copolymers, olefinic polymers or copolymers, such as, polyethylene, polypropylene or copolymers of polyethylene or polypropylene with other olefinic monomers, polyurethanes, acrylic polymers or any other plastic materials which can be foamed.
In a preferred embodiment the polymer or copolymer composition is comprised, exclusively or as part of a blend, of an alkylene aromatic polymer or copolymer. An alkylene aromatic polymer according to the present invention is preferably comprised of monovinylidene aromatic monomers. Preferred examples thereof are styrene, alkylstyrenes, for example, styrenes having at least one C1-C4 alkyl substituent located on the aromatic ring and/or on the vinylidene residue, and halogenated styrenes. Specific examples are styrene, xcex1-methylstyrene, a styrene having at least one alkyl and/or halo substituent on the aromatic ring, such as o-methylstyrene, m-methylstyrene, p-methylstyrene, or the corresponding ethylstyrenes or chlorostyrenes, vinylxylene or mixtures thereof. An alkylene aromatic copolymer is comprised of alkylene aromatic monomers and other polymerizable monomers, wherein the alkylene aromatic monomers preferably are present in an amount of at least 50 percent by weight, more preferably in an amount of at least 70 percent by weight, and most preferably in an amount of at least 85 percent by weight based on the weight of the total monomers. The other polymerizable monomers preferably are selected from acrylic-monomers, for example, acrylonitrile, acrylic acid, and acrylic esters such as butyl acrylate, or from mono- or poly-unsaturated olefinic monomers, for example, ethylene, propylene, butadiene, and isoprene, or mixtures thereof.
According to a further preferred embodiment of the present invention the polymer or copolymer composition is comprised, solely or as part of a blend, of an olefinic polymer, for example, a polymer comprised of olefinic or poly-olefinic monomers. Preferred examples are homopolymers of ethylene or propylene, copolymers of ethylene or propylene with one or more C4-C8 xcex1-olefins and inter- or copolymers of ethylene or propylene and at least one non-olefinic comonomer, wherein the olefinic monomers preferably are present in an amount of at least 50 percent by weight, more preferably in an amount of at least 70 percent by weight, most preferably in an amount of at least 85 percent by weight based on the weight of the total monomers. The non-olefinic polymerizable monomers preferably are selected from acrylic monomers, monovinylidene aromatic monomers or mixtures thereof.
The blowing agent in the method of the present invention can be selected from any known blowing agent suitable for the respective polymer, for example, from aliphatic or cycloaliphatic compounds including hydrocarbons, ethers, lower alcohols, halogenated hydrocarbons, especially partially halogenated hydrocarbons, and xe2x80x9cinorganicxe2x80x9d blowing agents such as water, carbon dioxide, nitrous oxides such as NO, NO2 and N2O, nitrogen, ammonia, noble gases such as argon and air, or mixtures thereof. Inorganic blowing agents can also be produced in situ by adding chemical compounds to the composition which decompose and generate gas, such as known typically in the art, for example, azo-type compounds for the generation of N2, ammonium compounds of the generation of NH3 and mixtures of carbonates and acids for the generation of CO2.
The blowing agent is generally used in an amount of from 0.5 to 25 weight percent, preferably of from 1 to weight percent based on the total weight of the foamable composition.
Examples of suitable aliphatic or cycloaliphatic compounds are ethane, ethylene, propane, propylene, butane, isobutane, butylene, isobutene, pentane, neopentane, isopentane, cyclopentane, hexane, heptane, cyclohexane and mixtures thereof. Suitable examples of ethers are dimethyl ether (DME), methyl ethyl ether, or diethyl ether. Suitable examples of lower alcohols are methanol, ethanol, propanol, isopropanol, butanol, pentanol, hexanol and mixtures thereof, wherein ethanol is preferred. Among the xe2x80x9cinorganicxe2x80x9d blowing agents, carbon dioxide or carbon dioxide/water mixtures are preferred. Examples of partially halogenated hydrocarbons are chloroethane, chlorodifluoromethane (R-22), 1-chloro-1,1-difluoroethane (R-142b), 1,1,1,2-tetrafluoroethane (R-134a), 1,1,2,2-tetrafluoroethane (R-134), 2-chloro-1,1,1,2-tetrafluoroethane (R-124), pentafluoroethane (R-125), 1,1-difluoroethane (R-152a), 1,1,1-trifluoroethane (R-143a), 1-fluoroethane (R-161), difluoromethane (R-32), 1,1,1,3,3-pentafluoropropane (HFC-245 fa), 1,1,1,3,3-pentafluorobutane (HFC-365 mfc) and mixtures thereof.
Preferable in all cases are blowing agent compositions which have no ozone depletion potential (ODP), namely fluorinated alkanes, inorganic blowing agents, alcohols, hydrocarbons, ethers or combinations thereof. Particularly suitable, for example, for alkylene aromatic polymers and copolymers, or for olefinic polymers and copolymers, are blowing agent compositions composed primarily of carbon dioxide, and mixtures of carbon dioxide with water or ethanol or isopropanol or dimethyl ether or mixtures of two or more of these. Compositions based on (i) 1,1,1,2-tetrafluoroethane, (ii) 1,1,2,2-tetrafluoroethane, (iii) 1,1-difluoroethane, (iv) mixtures of two or more of these, or (v) mixtures of each compound or mixture with ethanol or isopropanol or dimethyl ether or water or carbon dioxide or mixtures of two or more of these are also particularly suitable in the practice of the present invention. Additionally, compositions based on dimethyl ether and mixtures of dimethyl ether with water or ethanol or isopropanol or carbon dioxide or mixtures of two or more of these are also suitable in the practice of the present invention. Other suitable blowing agents are hydrocarbons, such as propane, butane, pentane or mixtures thereof. Furthermore, mixtures of suitable hydrocarbons with dimethyl ether, carbon dioxide, and partially halogenated hydrocarbons are also suitable in the practice of the present invention.
When preparing the multilayer foam according to the present invention it is in some cases preferred that a control of the foam temperature based on the thermodynamic nature of the blowing agent formulation is carried out in steps (d) and (e) as described above. In some cases, when using a blowing agent formulation comprising a blowing agent or a mixture of blowing agents which exhibits a low evaporative cooling capacity during the foaming process, an accelerated cooling during step (e) of the process is preferred to obtain a product having excellent quality. Examples of suitable low evaporative cooling capacity blowing agents are carbon dioxide, nitrogen, noble gases, nitrous oxides and air. In contrast, when using a blowing agent formulation comprising a major amount of a blowing agent or a mixture of blowing agents which exhibits a high evaporative cooling capacity during the foaming process, it was found that an additional transfer of heat to the composition during step (d) was beneficial in some cases to obtain satisfactory results.
The additional transfer of heat can be accomplished, for example, by also heating the composition to a temperature above the foaming temperature as it passes through the extrusion die, or by heating the environment directly adjacent to the extrusion die face to such an extent that a sufficiently high foam surface temperature is maintained throughout the point in time at which the expanding foam structures coalesce. Said heating can be accomplished in various ways such as known in the art, all such that (a) the extrusion die structure, or (b) the individual foam strand surfaces are heated above the bulk foaming composition temperature.
Examples of blowing agents having a high evaporative cooling capacity are hydrocarbons, ethers, partially halogenated hydrocarbons, water or mixtures comprising as a major component at least one of such blowing agents.
It should be noted that in many cases the temperature control as described above is not an essential feature to obtain multilayer foam products having an excellent quality. However, especially when using a polymer or copolymer composition comprising an alkylene aromatic polymer or copolymer and when preparing a foam having at least one layer comprising a plurality of individual strands, the temperature control may be of great importance. A detailed description of the temperature control may be found in the co-pending application by the same applicants having the title, xe2x80x9cLow Density Strand Foams.xe2x80x9d
Apart from the blowing agent, the composition may also comprise further additives, such as nucleating agents, extrusion aids, antioxidants, flame retardants, colorants, or pigments. Examples of pigments are carbon black or titanium dioxide or graphite and other compounds known in the art which may further enhance the thermal insulation properties of foamed products.
According to the present invention, foam structures comprising a plurality of layers and having a large cross-section can be produced. The present invention is particularly suitable for producing foamed products having excellent thermal insulation characteristics with cross-sections of at least 150 cm2 or more, more preferably at least 500 cm2 or more. It should, however, be noted that also foams having smaller cross-sections can be produced with the same advantages as previously described. The cross-sectional area of individual foam layers can be varied over a broad range. Usually the cross-section of a single foam layer is in the range of from 1 cm2 up to 500 cm2, preferably of from 2 cm2 to 250 cm2.
By the process as described above a cellular foam product can be obtained. In some cases, the cells of at least one foam layer may have an anisotropic shape, that is, the cell size is different when measured in different orientations (horizontal, vertical and extrusion).
Surprisingly, it was found that the polymer or copolymer composition can be substantially free from adhesive additives, for example, ethyl vinyl alcohol copolymer (EVA), ethyl acrylic acid copolymer (EAA), or other adhesive agents known in the art. Preferably, the adhesive additives are present in an amount lower than 5 weight percent, more preferably in an amount of lower than 1 weight percent based on the total polymer or copolymer composition. Most preferably, the composition is free from adhesive additives.
The use of adhesive additives, while not necessary for the easy production of, the final properties of or the excellent performance of the foams of this invention, may, however, be employed additionally such as is known in the art. Delivery methods of such adhesive additives would include mixing in the polymer, addition during any stage of the extrusion process or application of suitable adhesives during or after passage through the extrusion die, all such as known in the art.
According to the present invention, the polymer and blowing agents preferably are mixed in a mixer or in the extruder, and afterwards the mixed composition is cooled down prior to extrusion. Then, the composition, usually being in the form of a gel, is forced through a plurality of die orifices, preferably through a die plate comprising a plurality of openings, where at least one such opening is so designed that a monolithic foam layer extending across the whole breadth of the foam is produced. The individual foam layers can be planar or corrugated, discontinuous, for example, having ends, or continuous, for example, being in the form of a circle.
When a foam having at least one layer comprised of a plurality of individual strands is produced, the composition preferably is forced through a die plate comprising a plurality of small holes in a suitably desired spatial arrangement, for example, in an oscillating form such as a sinus wave, honeycomb, square, saw tooth or triangular saw tooth wave pattern.
According to the present invention it is preferred that the first contact between individual foam layers occurs after the departure from the respective die orifices. Further, it is preferred that the process according to the present invention is not carried out in a closed molding box but rather that a free molding of the foam product is conducted, optionally in an intermediate space between two plates. Furthermore, it is preferred in the process according to the invention that the total area of the openings of the dies used is xe2x89xa610 percent, more preferably xe2x89xa65 percent, and most preferably xe2x89xa63 percent in relation to the cross-section transverse to the direction of extrusion of the resulting product.
In order to enhance the self-adhesion between individual layers or strands, the foaming temperature is above the glass transition temperature or the melting temperature of the polymer or copolymer. The difference in temperature is preferably 1xc2x0 C. to 50xc2x0 C., more preferably 2xc2x0 C. to 40xc2x0 C., and most preferably 5xc2x0 C. to 30xc2x0 C. Furthermore, it is preferred that the theoretical diameter of an individual strand (T) or the theoretical thickness of an individual layer (T) is at least as high as the distance between the orifices (D) or the distance between the slits (D). The ratio T/D which is designated as cohesion ratio (C) is thus preferably greater than or equal to 1, more preferably the cohesion ratio is greater than or equal to 1.2. The theoretical diameter of each strand or thickness of each layer can be calculated based on the foam volume expansion and the line speed determination. The foam volume can be calculated from mass balance and foaming temperature. It can also be deduced from the foam density measurement.
Further, the present invention comprises using conventional co-extrusion technology to co-extrude multiple polymer/blowing agent systems. The nature of each polymer or each blowing agent can be different. It is, however, important that the foaming temperature is above the glass-transition temperature or the melting temperature of each polymer or copolymer, and preferably the cohesion ratio is equal to or larger than 1.
When producing a multilayer foam comprising multiple polymer/blowing agent systems, the present invention comprises co-extruding a first polymer or copolymer through at least one slit of the multiorifice die and a second foamable polymer or copolymer through at least one second slit of the multiorifice die, whereby a foam composition is produced comprising at least one layer of the first polymer or copolymer foam and at least one layer of the second polymer or copolymer foam.
For example, the outer layers of a foam composition can be made of an olefinic polymer such as polypropylene with a hydrocarbon blowing agent, for example, butane, and the inner layer or layers are made with an alkylene aromatic polymer, such as polystyrene, and, for example, CO2 and/or hydrofluorocarbons as blowing agent. The resulting composite foam offers a very good insulation value, thanks to the extruded polystyrene foam core, and a high service temperature and good chemical resistance due to the performance of the polypropylene outer shell.
A polymer or copolymer multilayer foam, for example, an alkylene aromatic and/or olefinic polymer or copolymer foam, which is obtainable by the method of the present invention preferably has a density of up to 150 kg/m3. More preferably, the density is from 16 to 80 kg/m3.
The expansion of the foam after leaving the extrusion die can be carried out in a normal atmospheric environment. For the expansion, however, also a sub-atmospheric environment, for example, partial vacuum, a super-atmospheric environment, for example, overpressure, and atmospheres having various gaseous compositions, for example, an elevated CO2 content, can be selected, such as is known in the art.
The foamed product may be transported by various conveying devices as known in the art, for example, continuous belts, or rollers. These devices may additionally comprise means for temperature control as appropriate.
Further, the foamed product which is produced according to the present invention is suitable for use in processes involving devices for reducing density after initial foam expansion, for example, infrared ovens, steam ovens, hot air ovens, or combinations of such devices.
If desired, the foamed product can be subjected to post-treatment procedures, such as embossing, remelting and other techniques known in the art to modify the outer surface of the foamed product. It should, however, be noted that according to the present invention a product with a smooth, uniform surface substantially equivalent to current monolithic foams is obtained, which in many cases does not require any surface-modifying operations.
By using the present invention, a foam having a large cross-section and a low density can be produced. This invention is suitable for production of foams with an average cell size ranging from 25 to 3000 micrometers (xcexcm), preferably 50 to 2000 xcexcm, and more preferably 100 to 1500 xcexcm. This foam presents a superior thermal insulation performance and dimensional stability in comparison with the foams of the prior art.
The multilayer foam according to the present invention preferably has an improvement in the thermal conductivity by at least 1.5 percent, more preferably by at least 3 percent, and most preferably by at least 5 percent over a monolithic foam having the same density and thickness and produced from the same polymer/blowing agent composition. xe2x80x9cMonolithicxe2x80x9d as used herein means a single, integral unit.
A preferred feature of the process and resulting foam according to the present invention is that at least one layer of the foam structure is composed of a plurality of closed-cells, preferably a closed-cell content as measured by ASTM D 2856-90 of at least 90 percent, and more preferably at least 95 percent. More preferably, all layers of the foam product are composed of a substantially closed-cell foam.
A further advantage of the foam according to the present invention over the prior art foams is that continuous layers in the direction of extrusion can be obtained. Further, it is preferred that according to the present invention the interface between individual strands or layers substantially does not exhibit any zones or only very small zones having an increased foam density.
A further preferred feature of the foam according to the present invention is that a minimum tensile strength, for example, as measured by ASTM D-1623 of at least 20 kPa, more preferably of at least 100 kPa and most preferably 250 kPa or higher is obtained. Further, the shear strength of the foamed product, for example, as measured by ASTM C-273 is preferably at least 10 kPa, more preferably at least 100 kPa and most preferably 250 kPa or higher.
A further preferred feature of a foam according to the present invention is that there are substantially no voids when cutting the foam perpendicular to the direction of extrusion. Preferably, the void area is less than 1 percent, and more preferably less than 0.5 percent based on the total cross-section of the foam. Most preferably, there are no voids at all.
The substantially voidless multilayer foams have at least substantially equal water vapor permeability and water diffusion resistance as comparable monolithic foams.